Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In both wireline and LWD environments, it is often desirable to construct an image of the borehole wall. Among other things, such images reveal the fine-scale structure of the penetrated formations. The fine-scale structure includes stratifications such as shale/sand sequences, fractures, and non-homogeneities caused by irregular cementation and variations in pore size. Orientations of fractures and strata can also be identified, enabling more accurate reservoir flow modeling.
Borehole wall imaging can be accomplished in a number of ways, but micro-resistivity tools have proven to be effective for this purpose. Micro-resistivity tools measure resistivity of the borehole surface on a fine scale. The resistivity measurements can be converted into pixel intensity values to obtain a borehole wall image. However, oil-based muds can inhibit such measurements due to the variability of impedance in the mud surrounding the tool.
U.S. Pat. No. 6,191,588 (Chen) discloses an imaging tool for use in oil-based muds. Chen's resistivity tool employs at least two pairs of voltage electrodes positioned on a non-conductive surface between a current source electrode and a current return electrode. At least in theory, the separation of voltage and current electrodes eliminates the oil-based mud's effect on voltage electrode measurements, enabling at least qualitative measurements of formation resistivity based on the injection of a current excitation signal and the subsequent measurement of the voltage drop across the formation. The voltage drop sensed between the voltage electrodes is amplified, conditioned, acquired, and used with a measured current flow to calculate an estimate of formation resistivity in front of the pad.
The implementation of a differential amplifier to measure a signal corresponding to the voltage drop in the formation encounters several obstacles. These obstacles include: the limitations inherent in the circuitry, the interactions of the sensor pad with the surrounding environment, the properties and standoff thickness of the mud, and the tilt angle of the pad relative to the formation. Among other things, these obstacles create a vulnerability to measurement error due to a common mode signal at the voltage electrodes relative to the amplifier reference ground. The above-named obstacles contribute variability to the voltage dividers defined by the input impedance of the amplifier and the impedances between the voltage electrodes and the formation. The finite input impedance of the amplifier circuit allows a small amount of current flow through these variable voltage dividers, converting the common mode voltage into a differential voltage component at the voltage electrodes.
Significant effort has been made to minimize the effects of common mode voltage. For example, one proposed method of reducing the common mode voltage relies on isolating the current source transmitter circuitry from the reference ground of the amplifier. However, attempts to provide a high-impedance isolation for the amplifier have been largely unsuccessful, and the residual sensitivity of the measurement circuitry to the common mode voltage remains too high to gather accurate measurements in boreholes having an oil-based mud.
The drawings show illustrative invention embodiments that will be described in detail. However, the description and accompanying drawings are not intended to limit the invention to the illustrative embodiments, but to the contrary, the intention is to disclose and protect all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.